edit the question with an "update" message? (I personally don't like the way that looks.)

post a comment under the question? For example:

Now that we know it's a peace lilly, I've added a tag to make it easier for future readers to locate the question.

leave the question alone, and let the OP add the tag if they so desire?

do something else?

I have the same question regarding certain "diagnosis" questions. Should I ask it separately or would the same general rule apply? For instance, if a plant has been diagnosed as having spider mites, adding that tag could definitely be helpful to the community.

@J.Musser I have no problem at all with this being a duplicate. In fact, finding duplicates is something I support! Interestingly, even though the consensus was the same in both, that we should go back and apply a tag, neither of us did it to the example questions we used! I'll do it now!
– Sue Saddest Farewell TGO GLSep 11 '17 at 21:22

@J.Musser Interesting. That one's a tiny bit different but not really enough. I added my close vote but I assume Alina gets a chance to decide if she wants to edit it.
– Sue Saddest Farewell TGO GLSep 12 '17 at 0:48

I like the way you wrote this, and will begin adding tags when appropriate. In fact, if you'd like to do the same, it would be helpful too! If they end up not looking right, anyone in the community can certainly edit them back out. Thanks for taking the time to answer me!
– Sue Saddest Farewell TGO GLDec 1 '16 at 22:32